1. Field of the Invention
The present invention relates to a semiconductor memory device having electrically programmable nonvolatile memory cells with floating gates, and more particularly to a method of injecting electrons into the floating gates.
2. Description of the Related Art
Among the known types of electrically programmable nonvolatile semiconductor memory devices are electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and flash memory. Each of the many memory cells in a nonvolatile semiconductor memory device of one of these types has a high-resistance floating gate for storing information. Each memory cell is in one of two states, depending on whether electrons have been injected into the floating gate, and thus stores a one-bit digital value, which can be sensed as a high or low signal value. Injecting electrons into the floating gate is equivalent to writing data in the memory cell, and is also referred to as programming the memory cell.
Two general methods of injecting electrons into the floating gate of a memory cell are known.
(1) The channel hot electron (CHE) injection method generates high-energy electrons, also referred to as hot electrons, by passing a high current between the source and drain of the memory cell, and injects the hot electrons into the high-resistance floating gate.
(2) The Fowler-Nordheim (FN) tunneling method applies a high voltage between the gate and drain of the memory cell, creating a strong electric field that enables current to tunnel into the floating gate.
FIG. 11 illustrates the programming of a memory cell by the conventional CHE injection method.
To program the memory cell M81 in FIG. 11, a potential Vg of twelve volts (12 V) is supplied from a power supply to the control gate electrode (G), a potential Vd of six to seven volts (6 V to 7 V) is supplied to the drain electrode (D), and the source electrode (S) is grounded. A high current IDS flows from the drain electrode to the source electrode, generating hot electrons, some of which are injected into the floating gate (FG) 82. The current flow is switched on and off by a control circuit (not shown). Basically, the current is switched on, allowed to flow for a time sufficient to inject the necessary quantity of charge into the floating gate 82, then switched off. Thus conventional CHE injection takes place in a single current pulse, during which current flows at a steady high rate from drain to source.
FIG. 12 illustrates the programming of a memory cell by the FN tunneling method.
To program the memory cell M91 in FIG. 12, a potential Vg of ten volts (10 V) is supplied from a power supply to the control gate electrode (G), a potential Vd of minus eight volts (−8 V) is supplied to the drain electrode (D), and the source electrode (S) is left floating. The high voltage (18 V) between the control gate and drain electrode enables current to flow by a tunneling effect, injecting electrons into the floating gate (FG) 92. The high voltage is applied between the control gate and drain as a single voltage pulse, under control of a control circuit (not shown). Tunneling current flows at a low rate, so the pulse duration is correspondingly long.
Thus, when data are written in a memory cell in a conventional nonvolatile semiconductor memory device by injecting electrons into the high-resistance floating gate, either CHE injection is performed with one high current pulse, or FN tunneling current is generated by one high voltage pulse.
The conventional method of CHE injection described above requires considerable current. If the memory device has an array of memory cells with floating gates arranged in a row-column matrix, and if the control circuit injects electrons into a plurality of memory cells simultaneously, the power supply circuit must have a correspondingly high current capacity. A resulting problem is that if an external power supply circuit is used to program many devices at once, a large power supply is needed. A similar problem occurs if the power supply circuit is a voltage-boosting circuit integrated into the memory device; to provide the needed current capacity, the power supply circuit must be relatively large in scale, taking up much space in the layout of the memory device.
The FN tunneling method requires relatively little current, and can inject electrons simultaneously into a plurality of memory cells without the need for a large power supply circuit. FN tunneling, however, takes more time than CHE injection. A memory cell of the FN tunneling type therefore has a longer write access time than does a memory cell of the CHE injection type. Furthermore, to write data into one particular memory cell in an array of memory cells of the FN type, a high voltage must be applied for a relatively long period of time between the control gate and the drain electrode of the memory cell. Compared with CHE injection, FN tunneling therefore requires more complicated control circuitry (including memory cell selection circuitry and a voltage-boosting circuit).